In telecommunication networks there are two methods of numbering, that is to say of identifying subscriber lines, switching centers or network areas: closed numbering and open numbering. In the case of closed numbering, the identity number of the switching centers is combined with the ordinal number of the associated subscriber lines, that is to say the identity number of the switching center is a fixed component part of the subscriber call number, it is contained therein in concealed form. In the case of open numbering, the numbering areas of the network are identified by a sequence of digits which are to be dialed in addition to the subscriber call numbers when setting up a connection between the network areas (Bergmann, Lehrbuch der Fernmeldetechnik [Textbook of telecommunication engineering], Volume 1, 5th edition, Berlin 1986, page 206). Heterogeneous networks in which call numbers are signaled by the open method in individual areas and by the closed method in other areas are also known.
In private and public ISDN networks, when setting up a connection the call number of the calling (A) subscriber is signaled to the called (B) subscriber. The call number of the A subscriber is required for implementing various facilities, inter alia for call number indication on a display of the terminal equipment on the reception side (B), for call-back when free and for redialing by the B subscriber. In continuously open numbering networks, when setting up a telecommunication connection which is routed from an origin node, via transit nodes, to a destination node, the A call number is to be supplemented by the identity number of the A terminal equipment. This supplementing is required, for example, in order that all the dialed digits which are to be entered into the B terminal equipment in order to reach the A terminal equipment are indicated on the display of the B terminal equipment. If, on the other hand, the origin node and following nodes lie in an area of closed numbering, in the origin node the A call number is not to be supplemented by the identity number of the origin node or the identity number of the area of closed numbering.
If the telecommunication connection is routed out of the area of closed numbering from the origin node into an area of open numbering, which comprises for example a plurality of transit nodes, the signalling information of the connection to be set up is supplemented by a node-individual identity number in each node involved in the setup of the connection in the area of open numbering. Depending on the number of nodes involved in the setup of the connection and depending on the number of digits of the node-individual identity numbers, the call number information may in this case comprise many digits in addition to the call number part which is individual to the terminal equipment. The display of such a supplemented call number presupposes a corresponding dimensioning of memories which have to be accessed by the corresponding display control. It must be taken into consideration here that, for example for implementing the facilities "call-back when free" and "redialing by B subscriber", use of the identity numbers supplemented by the identity numbers of the nodes involved in the original setup of the connection would lead to the following connection being routed via the nodes via which the original connection was routed. However, this would only be appropriate if the network were not to permit a routing any more favorable for call handling at the time at which the following connection is set up than at the time at which the original connection was set up.
The described problems in the case of the prior art could be avoided if a line were to be installed from the area of closed numbering to the destination node and the connection would be set up over this line. However, this would require considerable expenditure, in particular if origin node and destination node are at locations which are far apart. This possibility is virtually ruled out if the origin node and destination node are in different continents (Europe/USA).
The product publications of Siemens AG "Hicom 300 System Produkt Data" [Hicom 300 Systems Product Data], ordering No. A19100-K3161-430-X-7600, pages 55/56 and "Private Kommunikationssysteme, Hicom schafft Verbindungen" [Private Communication Systems, Hicom Creates Connections], ordering No. A19100-K3160-A302, page on "Hicom 300 Networking" discloses a communication system ("Hicom 300") which can form a network node in an area of open numbering or closed numbering. In such a network, signalling information on telecommunication connections to be set up is formed and an identity number designating the concealed area (receiving the origin node) and the call number of the telecommunication terminal equipment on the transmission side are inserted into this information. In the signalling information, the call number of the telecommunication terminal equipment on the transmission side is supplemented by identity numbers of the nodes involved in the setup of the connection, so that the disadvantages described above arise.